1. Field of the Invention
The present invention relates to an optical access network system that allows a service provider and a subscriber to communicate by using a CDM (Code Division Multiplexing) system in a PON (Passive Optical Network).
2. Description of Related Art
An optical access network system constituted by connecting a service provider (also called the ‘center’ hereinbelow) and a plurality of subscribers (also called ‘users’ hereinbelow) via a PON has attracted attention. In the subsequent description, a service provider-side device is also called an optical line terminal or OLT and a subscriber-side device is also known as an optical network unit or ONU.
A PON is a network that splits a single optical fiber transmitting line into a plurality of optical fiber transmitting lines by connecting a star coupler constituting a passive element midway along the optical fiber transmitting line and which connects a plurality of optical network units in a star shape with the star coupler at the center (See Yokota et al. ‘Optical Access System ATM-PON’ Oki Electric Research and Development “OKI DENKI KENKYU KAIHATSU”, 182nd Edition, Vol. 67, No. 1, April 2000, for example). By adopting a PON as the network linking the center and users, the optical fiber transmitting line between the center and star coupler can be shared by a plurality of users, whereby equipment costs can be suppressed.
In a conventional optical access network system that utilizes a PON, users allocated to the respective transmitting lines are identified by adopting a time division multiplexing (TDM) system and controlling the time slots of the TDM signals (See Ian M. McGregor, et al. “Implementation of a TDM Passive Optical Network for Subscriber Loop Applications”, J. Lightwave Technology, Vol. 7, No. 11, November 1989, for example). Here, optical signals of different wavelengths are employed for signals from the user to the center (also called ‘uplink signals’ hereinbelow) and signals from the center to the user (also called ‘downlink signals’ hereinbelow). This is so that the uplink signals and downlink signals are identified on the basis of a wavelength difference because the uplink and downlink signals share one optical fiber transmitting line. The uplink and downlink signals are separated and multiplexed by an optical bandpass filter and signals between the respective users and the center are multiplexed and demultiplexed by the star coupler.
On the other hand, in an optical access network system that utilizes a PON, methods for transmitting uplink signals by means of WDM (wavelength division multiplexing) have also been investigated (See K. W. Lim, et al. “Fault Localization in WDM Passive Optical Network by Reusing Downstream Light Sources”, IEEE Photonics Technology Letters Vol. 17, No. 12, December 2005, for example). However, in order to increase the number of channels multiplexed (number of users here), the wavelength bandwidth that can be used is of a finite width and it is therefore necessary to make narrow the wavelength interval allocated to adjacent channels. In order to make the wavelength interval narrow in this manner, light-source wavelength stability is required and high equipment costs are required in order to secure this stability.
Therefore, the number of wavelengths used is not increased, the number of multiplexed channels is increased, and the transmitting capacity is desirably substantially increased. As one such method, a method that performs communications between a center and users by means of CDM transmitting has been investigated (See Japanese Unexamined Patent Application No. 2001-512919 (corresponding to WO 99/07087), for example).
The communication method disclosed in Japanese Unexamined Patent Application No. 2001-512919 is a method in which an electrical signal that is transmitted is encoded and multiplied and then up-converted to an RF (radio frequency) signal before being converted to an optical transmitting signal. Further, in the passive-type subscriber network disclosed in Japanese Unexamined Patent Application No. 2004-282742, a method in which the electrical signal to be transmitted is encoded and multiplied before being converted to an optical transmitting signal and transmitted is adopted and, in addition, a WDM system is also included and adopted. Here, a method that multiplexes and splits the transmitted optical signals and received optical signals by means of an optical circulator is used and the same wavelength is employed for the transmitted optical signals and received optical signals.
In the case of each of the devices disclosed in Japanese Unexamined Patent Application No. 2001-512919 and Japanese Unexamined Patent Application No. 2004-282742, in order to receive a signal, the receiving signal is multiplied by code synchronized with the transmitting side. A constitutional example of the receiver used for this purpose is disclosed in Rushikesh S. Kalaspurkar, et al. “Performance Evaluation of a Recurring State Dynamic Digital Matched Filter for DS-CDMA”, IEEE 2003. In the case of this receiver, an RF signal is converted to a baseband electrical signal before being A/D converted and the receiving signal is obtained through the generation of an auto-correlation waveform by the digital shift register and digital correlation computation device.
Because a CDM system is adopted for the PON, even when the number of wavelengths used increases, the number of multiplexed channels (corresponds with the number of users) can be increased.
However, executing an optical access method using PON that includes a step of encoding and multiplying a transmitted electrical signal, a step of up-converting the signal to an RF signal, and an A/D conversion step by means of an A/D converter produces the following problems. That is, when a case where high-speed signals (high bitrate signals) with a bit rate of 100 Mbit/s are sent and received is described as an example, there are the following difficulties. In this case, the diffusion rate that is required to encode a signal by means of code with a codelength (definition of codelength will appear subsequently) of 16 must be 1.6 Gbit/s at the lowest and, in order to permit up-conversion to an RF signal, a frequency that is on the order of eight times that codelength is required. Hence, a carrier wave of 12.8 GHz or more is required. Therefore, an A/D converter and a digital multiplier that performs code multiplication on the transmitted electrical signal must be allowed to operate at a speed of at least 1 Gbit/s or more. In their current form, an A/D converter and digital multiplier capable of operating at such a high speed are hard to obtain.
Furthermore, in the case of a conventional PON-based optical access network system, light of different wavelengths is used for the uplink and downlink signals in order to prevent the mixing of reflection noise that is produced in an optical connector or the like for a connection with an optical (de)multiplexer provided in the optical transmitting line. Hence, two types of light of different wavelengths are required each time the subscriber terminals are increased by one. Thus, the required number of light components with different wavelengths is very high.
Furthermore, in a PON-based optical access network system that adopts a CDM system, there is the problem that a portion of the transmitting signal transmitted from the ONU is reflected in the optical connector for the connection with the star coupler provided in the optical transmitting line and a portion of the transmitting signal is mixed with the receiving signal received by the ONU as reflection noise. Due to the noise component, a situation where the receiving signal cannot be received correctly by the ONU arises.
There is therefore a method that allocates different wavelengths to the signals transmitted from the ONU to the OLT and the signals transmitted from the OLT to the ONU. According to this method, there is a need to increase two-fold the types of the wavelengths to be allocated to the signals. When the fact that the wavelength resources that can be used in optical communication is finite is considered, an increase in the types of wavelengths used is undesirable.
Further, as means for reducing the intensity of the noise that is mixed with the receiving signals received by the ONU, the usage of an optical connector of small reflectance as an optical connector for a connection with the star coupler provided in the optical transmitting line may be considered. The optical connector has a constitution that makes an optical connection by bringing optical fiber end faces that are to be connected to face one another by performing polishing so that the end faces tilt away from the vertical face with respect to the direction of propagation of light. Such a constitution keeps the amount of attenuation of reflected light at or below 60 dB.
However, a low-reflectance type optical connector is highly expensive and usage thereof in order to connect an ONU, which is a user device, with the star coupler is associated with high equipment costs of the optical access network system, which is undesirable. Because the ONU, which is a user device, is required in a quantity equal to the number of users, the adoption of one high-cost part as the ONU and the required parts that accompany the ONU greatly affects the equipment costs. On the other hand, because there may be one OLT, which is a center-side device, the adoption of a high-cost part hardly affects the equipment costs. Further, although failure of the OLT, which a center-side device, means that the whole system no longer functions, this does not mean that the functions of the whole system fail when an ONU, which is a user device, breaks down. Hence, whereas the low costs of the parts constituting an ONU are an important requirement, the height of the functions of the parts constituting the OLT are an important requirement.
Therefore, an object of the present invention is first of all to provide a PON-based optical access network system that allows high-speed signals to be sent and received.
Furthermore, a second object is to provide a PON-based optical access network system that, even when the number of subscribers increases, does not require an increase in the number of wavelengths used as does a conventional optical system of the same type.
In addition, a third object is to provide a PON-based optical access network system with which there is no reduction in the S/N ratio of the receiving signal even when a reflected light component from the optical connector that is used to connect the star coupler and the ONU is mixed with the receiving signal of the ONU.